The present invention relates to a communication apparatus and more particularly to a communication apparatus in which one of low and high frequency bands is used for transmission and the other band is used for receiving.
Such conventional apparatus which has been used for an automobile radio communication system is arranged as shown in FIG. 1. An oscillator 1 generates a frequency which is 1/n of a transmission frequency f.sub.t and generally comprises a phase locked loop (PLL). A phase modulator (PM) 2 is adapted to PM modulate the output of the oscillator 1 with an audio input from a microphone 3. A multiplier 4 multiplies an output frequency of the modulator 2 with a number n to provide a transmission frequency f.sub.t. A circulator 5 feeds the output of the muliplier 4 which has been amplified by an amplifier (not shown) to an antenna 6 from which r.f. power is emitted. The antenna 6 carries out receiving simultaneously with transmission and the receiving frequency f.sub.R is equal to f.sub.t .+-..DELTA.f.sub.1.
A front end mixer 7 has an input to which a signal received by the antenna 6 is applied via the circulator 5. A multiplier 8 is identical with the multiplier 4. The output of the multiplier 8 is input to the other input of the mixer 7 so that the mixer 7 creates an IF frequency f.sub.IF =f.sub.1 by using the transmission frequency as a local frequency. A demodulator 9 is adapted to FM or PM demodulate the IF frequency signal. A reference numeral 10 designates a speaker.
The operation of the apparatus as described above will be described by way of a frequency of the U.S. personal radio communication service (hereafter referred to as PRCS).
In PRCS, at the first channel, the transmission frequency f.sub.t is 937,015 MHz, receiving frequency f.sub.R is 898,015 MHz, the frequency interval f.sub.1 =f.sub.IF is 39 MHz when transmission is made at a high band. Since one of the frequencies is used for transmission, the other frequency is used for receiving, switching of the frequency is necessary in one set. For example the transmission frequency f.sub.t is 898,015 MHz, the receiving frequency f.sub.R is 937,015 MHz when transmission frequency is in the low band.
The oscillator 1 generates a frequency which is 1/n of the transmission frequency f.sub.t and the oscillation frequency is changed at each high and low band transmissions. The modulator 2 PM modulates the oscillation frequency from the oscillator 1 on high band transmission. Since the PM modulation has 180.degree. modulation factor at maximum, a required modulation factor is provided by frequency multiplication by a multiplier 4.
On reception mode, the oscillation frequency of the oscillator 1 is multiplied by a multiplier 8 and the multiplied frequency signal is used for a local frequency signal of the mixer 7 as a non-modulated signal. This local frequency is an upper local frequency and the IF frequency is equal to the frequency interval f.sub.IF =39 MHz.
On low band transmission mode, the modulated signal which is obtained by the oscillator 1, modulator 2, multiplier 4 is emitted from the antenna 6. Since the receiving is made at high band, the output frequency of the multiplier 8 is injected into the mixer 7 as a lower local frequency. The IF frequency f.sub.IF is similarly 39.
Since the oscillation frequency of the oscillator 1 should meet the requirements of the channel separation (133 channel, 30 KHz separation in PRCS), the oscillator 1 generates different frequencies at every 30/n KHz. Accordingly if the oscillator 1 comprises a PLL, the internal comparison frequency becomes very low, so that SN ratio (noise level) and tracking performance at frequency change (lock up time) deteriorate.
Accordingly conventional apparatus has disadvantages that the comparison frequency is low so that SN ratio and the suppression of spurious signal is deteriorated since the frequency should be multiplied in the afore-mentioned arranged conventional apparatus. It is also inconvenient since the IF frequency and the frequency interval is fixed at 39 MHz.